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Exactly. With Multipath TCP, a single TCP connection can benefit from multiple interfaces, which is not the case with bonding techniques. See http://multipath-tcp.org/pmwiki.php?n=Main.50Gbps for measurements showing Multipath TCP aggregating 6 10 Gbps interfaces.

An important difference between using Multipath TCP versus other techniques for 3G/WiFi offload is that Multipath TCP would be a completely end-to-end solution. The handover is entirely controlled by the smartphone and does not require any cooperation from the network. Existing 3G/WiFi offload techniques assume that the network operator controls both the WiFi and the 3G network. This is sometimes the case, but even if your 3G provider as several WiFi access points, there are probably many more access points that are not controlled by the 3G operator and that Multipath TCP would enable you to use.

If you use Multipath TCP with a single interface through a router, it will behave like regular TCP (doing NewReno congestion control scheme)